"Please see that my brain is given to the NFL's brain bank," Duerson wrote in a suicide note.

Was Duerson, a defensive back on two Super Bowl champions, a victim of Chronic Traumatic Encephalopathy, a degenerative brain disease that has claimed the lives of other professional players?

Or was it perhaps due to his financial problems?

What will his brain tell us?

Duerson is the first player to hint that the knocks he took as a football player drove him to suicide. We also know he was one of more than 250 current and former athletes who have agreed to donate their brain and spinal cord to the study of CTE.

"Whatever Dave Duerson's brain tells us, he told the world plenty by what he did," said Dr. Mike Happel, a local neurologist. "We need more organ donors like him because we have so much to learn. What we've learned so far about CTE is due largely to the advances that have been made in Alzheimer's. I'm hoping the Duerson story will produce more organ donors. You can't have too many of those."

Unlike Alzheimer's, there are no specific markers or tests to detect CTE in a living athlete. CTE is diagnosed by studying brain tissue under a microscope after death.

Unlike Alzheimer's, which has a genetic cause, CTE deals more in brain trauma, going back to something the medical world called "dementia pugilistica," the "punch drunk" fighter.

In a 2009 study of 51 confirmed cases of CTE, 90 percent turned out to be athletes: 39 boxers, five football players, one professional wrestler and one soccer player.

"One of the major problems dealing with CTE is the risk factor," said Happel. "And that depends on the individual. When it comes to blows to the head, some are a higher risk than others. How many concussions does it take to cause permanent brain deterioration? When do you tell a football player he should give up the sport?

"When you're dealing with concussions, you're not only dealing with professional, college and high school football players, you're dealing with playground football, with 12 year olds. You're dealing with all kinds of contact sports."

There are no hard and fast rules. New evidence shows 85 percent of concussions require three weeks of recovery. That's a longer period than anything in the books for returning to play. It tells you how much more research is needed before we know that the current neurological tests correspond correctly to the state of the brain.

"We know that CTE is characterized by a number of neurological and physiological changes in the brain, including the buildup of an abnormal protein called tau," said Happel. "The protein builds up in places in the brain where it's not supposed to. It congregates in clumps and disrupts the brain's function."

On hearing the Duerson story, former San Francisco offensive lineman Randy Cross said, "it ought to terrify anyone that's played the game."

Happel recalled watching a recent television clip of former 49ers quarterback Steve Young recalling a sack he'd rather forget, "talking about being knocked out cold."

"You see stuff like that and you think of the impact it has on players who've been through it," Happel said.

A four-time Pro Bowl player, Duerson was a member of a panel that considered the claims of retired players on such things as the "88 Plan," a fund founded in 2007 to defray families' costs on caring for retired players suffering with dementia.

There's no telling how all of this might have told the former Notre Dame All-American, who played for the Super Bowl-winning '85 Bears and '90 Giants, that something going on inside of him was not right.

That we found out. One bullet told us.

His brain may tell us more.

His former wife, Alicia, is seeing to it that one of her husband's longtime wishes will be fulfilled.